A protection device, for example, is described in German Patent No. 40 22 078 A1. A tripping shaft that belongs to the switching mechanism and can be activated by a release is provided as a component of the protection device. The release can in turn be influenced by the current flowing over the switching device. If the switch contacts are separated immediately by the forces associated with such a high current flow, the switching mechanism is released directly by the opening operation of the switch contacts, and thus a complete shutdown is brought about. The switching mechanism is released by a displaceable guidance of the tripping shaft against a restoring force with a reduction in coverage between the latching surfaces provided on a latch and on the tripping shaft.
Furthermore, it is known that the gas pressure that occurs when the switch contacts are opened in the arc quenching chamber of a switching device can be used for immediate actuation of the switching mechanism. Therefore, a pressure seal is provided for the switching mechanism and is connected to the arc quenching chamber by a pipeline (European Patent No. 455,561 A1).
Furthermore, it is possible to consider using methods which are known in another context for controlling a switching device and make it possible to detect a movement of a switch contact. This includes in particular an analysis device that measures the capacitance between the switch contacts and generates from it a signal for contact opening. Furthermore, a relative movement of a switch contact can be detected by a drive organ which is connected to the movable switch contact and is provided with a reflector, with an optical waveguide in a stationary mount opposite it a slight distance away. On the end facing away from the reflector, the optical waveguide acts together with a light source and a receiving circuit for reflected light. However, such devices as described in German Patent No. 39 05 822 A1 are relatively expensive supplementary devices of a switching device that need their own power supply and for whose electronic analysis circuits hardly any space is available in the casings of compact switching devices. On the other hand, the devices that operate mechanically according to the aforementioned German Patent No. 40 22 078 A1 and European Patent No. 455,564 A1 require a special structural adjustment to the desired operation with forced tripping when the switch contacts are raised.
Another restriction on the protection devices described above is that they can be used only in combination with switching devices having a switching mechanism that can be controlled by automatic devices. This condition is generally met with power circuit-breakers and contactors, but not with isolating switches.